Present public telephone systems adapted to direct long distance dialing offer increasing opportunities for unauthorized toll calls to be made from private telephone systems connected thereto as the complexity and sophistication of such systems progress. One such problem encountered is the restriction of dual tone multifrequency telephones due to signal detection timing between the dial tone source and that of the private exchange. Another problem occurs when the controlling exchange is able to respond to signals prior to the beginning of the dial tone signal. Still other problems concern the recognition of dialed signals before dial tone commences and thereby bypassing the toll restrictor and the characteristics of some controlling exchanges to recycle after completion of a preceding call so that unauthorized calls are permitted to go through the trunk lines.